For the 2 2TB Western Digital Mybook I have it partioned for a 1Tb clone of my Western Digital external drive, 300Gb partion of a clone of my Macbook, and a 700GB partion for photos. Currently I have about 680/700GB on the photo portion, 500GB on the 1TB external drive along with 300GB of an iTunes library on that same drive, with a 170GB/300GB of my Macbook clone.

I know that I will surpass 1.5TB of photos in this upcoming year. Does it make sense to purchase just two new 3TB external drives to replace the 2TB's? Should I look into a storage array so I won't have to purchase a new HD twice?

As so often: KISS

- Plan your primary storage with enough headroom for growth so that it lasts at least a year. Thus, get a 3 TB as your primary storage for images & iTunes media files (yes you can save money by getting a faster drive for image and a slower one for iTunes media files but unless you get a RAID 0, the difference in cost between a 2 TB 7200 rpm USB 3 (or FW800 if don't have a USB 3-equipped MB) + a 1 TB 5400 rpm USB 2 drive compared to a 3 TB 7200 rpm USB 3 drive is not that large.

If you want a portable drive, getting a 2 TB 2.5" drive for all your primary data should still last you a while.

- Don't mix backup and primary storage on the same physical drive (I don't know if you do currently, as your description is incomplete).

- Have at least two backups or better two local + one off-site.

- I see little need to exclude any of my primary data from the TM backup (apart from some special cases like virtual machines or ripped optical media where the optical media already provides one backup). Sure, in your case 3 TB external + 300 (?) GB internal would call for a 4 TB TM disk but migrating backups is easier than migrating primary data (and, a 3 TB TM disk would probably be fine as well for some time).

- For non-TM backups, I usually repurpose older drives (and might partition them but that is the only time I partition). If you have two 2 TB, you can set-up two clones of your photos + iTunes media and with your 1 TB drive you can create a clone of your internal HDD. That leaves you just short of one small (300 GB) drive to end up with three backups.